Maybe Itachi couldn't get Eternal Mangekyo Sharingan by using Fugaku's Mangekyo Sharingan, only his brother's Sharingan was required, but that doesn't seem to be the case.
Itachi could take Fugaku's Mangekyo Sharingan to get Eternal Mangekyo Sharingan. However, he may choose not to do so or did not want an Eternal Mangekyo Sharingan. He has never been thirsty for power. He was not even so happy when he got Sharingan. He was actually sad about his friend's death.
Another possible reason is, he just wanted to pass his eyes to Sasuke so he gets the Eternal Mangekyo Sharingan. If Itachi made his Sharingan, Eternal Mangekyo Sharingan, it would become a problem for Sasuke to get an Eternal Mangekyo Sharingan.